Agglutination assays have been widely used for many years in the testing of sample liquids. Traditionally such testing was conducted on a solid surface such as a glass microscope slide, and the presence of an agglutinated particulate reagent observed either with the naked eye or with the aid of a microscope. Today, assay kits are commercially available in which the particulate reagent is provided in dry form pre-dosed onto an appropriate solid surface, for example a "test card". The sample liquid is applied to the card, for example, as a drop of serum. To conserve reagents and to facilitate handling of the test card without risk of contaminating the dried reagent, this reagent is usually located in a designated test area on the card, for example a clearly marked area of circular or other shape. It is usually necessary physically to agitate the sample liquid to encourage reconstitution of the dried particulate reagent. Thereafter the agglutination reaction can proceed and the test result can be evaluated by inspection of the test card after an appropriate time interval. Test devices of this type are taught, for example, in U.S. Pat. No. 3,770,383.
To reassure the user that the test card is performing satisfactorily it is desirable for a control to be provided. Typically this is a positive control which assists the user to appreciate the degree of agglutination (or its relative absence) indicative of a clearly positive test result. For example, in some assays this control is provided in the form of a liquid sample containing the analyte of interest, for example antibodies (e.g. in serum) raised against an infective agent of interest (such as H. pylori or Neisseria), or a preparation comprising antigens from the organism of interest (which may take the form of an antigen extract or a bacterial cell suspension). To maintain the integrity of this liquid control regent it must be stored at low temperature. The need for access to the refrigerated storage facilities severely limits the pratical situations under which such tests can be conducted.
Furthermore, in presently-available tests, reconstitution of the dried particulate reagent does not always occur effectively.